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DTC codes / GM codes / P1323

P1323 · GMLoss of High Resolution RPM Signal

P1323 on a GM vehicle means the powertrain control module (PCM) has lost the high-resolution RPM signal it normally receives from the ignition control module or crankshaft position sensor circuit. The PCM uses this fast-switching signal to time spark and fuel delivery precisely, so when the signal drops out or becomes erratic the computer flags the circuit as failed.

SystemPowertrain
Code typeGM specific
SeverityMedium
Fix difficultyIntermediate
AreaIgnition system and misfire

Symptoms you may notice

  • Illumination of the check engine light, often with no other noticeable symptoms
  • Intermittent engine stumble or misfire at idle or under light load
  • Hard starting or a crank-no-start condition when the signal is completely lost
  • Rough running and reduced power if the PCM substitutes a default timing value
  • Tachometer needle fluctuation or dropout on vehicles with an instrument-cluster tach
  • Possible stalling at low speed or during deceleration

Common causes, most likely first

  1. Faulty crankshaft position sensor or ignition control module high-resolution output
  2. Damaged or chafed wiring in the high-resolution signal circuit between the sensor and PCM
  3. Corroded or loose connector terminals at the crankshaft position sensor or ignition control module
  4. Excessive air gap or debris on the crankshaft reluctor wheel causing a weak signal
  5. Failed PCM driver circuit for the high-resolution input
  6. Poor engine ground or voltage supply to the ignition control module affecting signal quality
  7. Aftermarket ignition components or a recent repair that disturbed the sensor harness

How to diagnose and fix it

  1. Scan all modules and record freeze-frame data to confirm P1323 is active and not a history code from a past event
  2. Inspect the crankshaft position sensor and ignition control module connectors for corrosion, pushed pins, or loose terminals
  3. Check the high-resolution signal wire for continuity, shorts to ground, and shorts to voltage between the sensor and PCM
  4. Measure the sensor air gap and inspect the reluctor wheel for damage or metallic debris
  5. Verify good engine ground and battery voltage at the ignition control module before condemning any component
  6. Swap in a known-good crankshaft position sensor or ignition control module only after circuit tests pass
  7. Clear codes and perform a road test with a scan tool graphing high-resolution RPM to confirm the repair

What the high-resolution signal actually does

  • Many GM vehicles store P1323 as a history code after a low-voltage event or a stall, so always check whether the code is current before replacing parts.
  • A failing ignition control module can produce a clean low-resolution RPM signal while the high-resolution output drops out, which fools technicians who only watch the main tach signal.
  • Aftermarket remote starters and alarm systems tapped into the crank signal circuit are a frequent hidden cause of intermittent P1323 on older GM trucks and SUVs.
  • The PCM may run the engine using a default timing strategy when the high-resolution signal is lost, so the vehicle can feel almost normal while the code is set.
  • A weak battery or failing alternator that causes voltage dips during cranking can set P1323 without any fault in the sensor or wiring itself.

Can I keep driving with P1323?

You can usually drive the vehicle short distances to a repair shop if the engine runs smoothly, but avoid heavy acceleration, towing, or long trips because a complete loss of the signal can cause a stall or a no-start. If the engine is misfiring, stalling, or cranking without starting, stop driving and arrange a tow.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for GM vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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